[Feature request] Consider adding support of reference proxy

Đọc tài liệu iterator_facade được liên kết trong issue và xem lại phần tiếp nối từ issue 47 để hiểu các quy tắc suy luận iterator-category. Xác định việc hỗ trợ các iterator RandomAccess có tham chiếu proxy nên có nghĩa là gì, bao gồm cả việc có cần một proxy tag hay không, và xác minh rằng API kết quả không còn yêu cầu ghi đè iterator_category theo cách thủ công.

Continuing https://github.com/boostorg/iterator/issues/47.
Currently, iterator_facade does not allow to have RandomAcess iterator with reference proxy (aka std::vector<bool>::iterator::reference) .

It looks like that the only thing that prevent this - is iterator-category deduction/override rules https://www.boost.org/doc/libs/1_68_0/libs/iterator/doc/iterator_facade.html :

iterator-category(C,R,V) :=
   if (C is convertible to std::input_iterator_tag
       || C is convertible to std::output_iterator_tag
   )
       return C

   else if (C is not convertible to incrementable_traversal_tag)
       the program is ill-formed

   else return a type X satisfying the following two constraints:

      1. X is convertible to X1, and not to any more-derived
         type, where X1 is defined by:

           if (R is a reference type
               && C is convertible to forward_traversal_tag)
           {
               if (C is convertible to random_access_traversal_tag)
                   X1 = random_access_iterator_tag
               else if (C is convertible to bidirectional_traversal_tag)
                   X1 = bidirectional_iterator_tag
               else
                   X1 = forward_iterator_tag
           }
           else
           {
               if (C is convertible to single_pass_traversal_tag
                   && R is convertible to V)
                   X1 = input_iterator_tag
               else
                   X1 = C
           }

      2. category-to-traversal(X) is convertible to the most
         derived traversal tag type to which X is also
         convertible, and not to any more-derived traversal tag
         type.

The simplest solution for now, seems to override manually iterator_category in Derived class.

Maybe add some proxy tag? Like:

class node_iterator
	: public boost::iterator_facade
	  <
		node_iterator
		, std::pair<int, int>
		, boost::bidirectional_traversal_tag
		, boost::iterator_facade::proxy_reference<std::pair<int, int>>
	  >

And do not downgrade iterator-category, if proxy_reference occurs.